This page describes how I solved the THX subwoofer crossover frequency issues while using a Marantz AV-9000 THX Ultra preamp in my home theatre stereo system with my own original subwoofer design, by adding a new low pass filter with a front end mixer. It works by connecting all of the front channels from the Marantz AV-9000, left, right, center and bass via splitter cables to the four inputs of the mixer on the new filter. This enables the subwoofer to operate with a single full range mono channel that can be filtered as required with out regard for anything that may have been done to the signal within the DSP processing of the Marantz AV-9000.
Rev B Active Filter
The main new requirement of the Rev B version of the active filter is to provide a four channel mixer at the input as described above. To enable this I have changed the design as shown in the diagram below. The first stage (section A in the diagram) is a four channel inverting buffer & mixer with a 47Kohm input impedance. This impedance gives a more even balance with the inputs of the power amps that will be connected in parallel from the preamp output for the various channels, whereas the 3k9 input impedance of the Rev A version was exclusively connected to a single preamp output. The outputs of the mixer channels are connected together and then connect to the input of the second stage (section B in the diagram) which is the low pass Sallen & Key filter. This arrangement is better that the Rev A version because it means that the filter is working with the full line level input rather than a possibly reduced level from the gain stage which will improve the signal to noise ratio. Rather than using cabled rotary switches for the filter settings, I used individual DIL switches soldered directly to the board. This has the advantage of being able to close multiple switches at the same time to make much bigger capacitor values for very low crossover frequencies and to mess around with the relationship of C1 to C2 to give different Q factors which will result in different responses other than Butterworth etc. Also it is much quieter as there are no cables to pickup any noise. Of course there is the obvious disadvantage that it is not easy to change the settings, but once the setting is found, you will never ever need to change it again for any given setup. This means there will be no electrical noise from the switches over time and also that there is no need to worry about making the seal where the switch passes through the cabinet airtight. When I was setting up, I just had some temporary long cables to connect the filter board so that it could hang out of one of the bass reflex ports to give access. When the setup was done, I fitted it into it's final place forever. If you want to, you could still add some other kind of switch with external access if needed. The third and final stage is the variable gain inverting buffer (section C in the diagram) that has the gain control and connects directly to the power amp input.
The table below shows a selection of switch settings for various crossover frequencies from 12Hz to 122Hz for a Butterworth response where R1 = R2 and C1 = 2 x C2. The mathematics that describe the filter are shown here. The place in the table where the frequency goes in the order of 20, 22, 21 is correct, its just that with the capacitor values getting larger, the change in resistor value actually overlaps the capacitor values a bit. For the very large capacitor values at very low frequencies, there is no value in changing the resistor value from 548K at all! If you want to, you can use the formula shown to workout any combinations not shown, but remember this formula only works for a Butterworth response where R1 = R2 and C1 = 2 x C2. The overall response of my particular subwoofer with the crossover frequency set to 17Hz is described here. With this modification to my subwoofer, it now works just fine with THX! The complete circuit and layout of the new board is shown here.
If you would like to make a high quality active low pass subwoofer filter like this for yourself, this section has everything you need to know! I made mine using the the Press-n-Peel system developed by Techniks. This is an easy system that can be used to print the circuit with a laser printer and literally iron it onto the copper board. The complete process for this is described on my 8051 Microcontroller page or you can look at other Press 'n' Peel links on the web. You will need two prints for the design, one for the copper tracks here, and one for the component side here. These are pdf files that will print the images in the correct size. Below you will find a list of components you will need and some example links of where you can get them from. As there are so few components, I didn't bother numbering them R1, R2 etc. but it is easy to work out. The numbers in brackets are the quantities required. If you have any questions, let me know.